1. Field of the Invention
The present invention relates to a magnetic reproduction apparatus having a magnetoresistance effect head (MR head). More particularly, this invention is concerned with a magnetic reproduction apparatus in which electromigration occurring in an MR head for reproducing information recorded on a magnetic recording medium is suppressed.
2. Description of the Related Art
In magnetic recording/reproducing apparatuses for recording or reproducing information on or from a magnetic recording medium, such as a magnetic disk unit or magnetic tape unit, a magnetic recording/reproducing apparatus having a composite head composed of a winding-type thin film magnetic head (inductive head) used for recording on a recording medium and an MR head used for reproduction has been put to practical use.
An MR head included in such a composite head detects a leakage magnetic field induced by a reversal of magnetization in a recording medium on the basis of the fact that the resistance of an element varies depending on an external magnetic field. A current termed a sense current is fed to the element when it is used. A reproduced output of the MR head is, unlike that of an inductive head, independent of the speed of the head in relation to a recording medium, is acquired as a relatively large value proportional to a magnetic flux density, and is therefore very useful in reproducing data from a high-density recording track. When the sense current flowing into the element in the MR head is increased, a large reproduced signal can be acquired irrespective of the relative speed of the head in relation to a disk. For this reason, the MR head is favorably adopted for a high-density magnetic disk unit. Due to an increasing demand for a larger-capacity magnetic disk units to be used as external storage units for computers, the MR head has been widely adopted in recent years.
However, as far as a magnetic reproduction apparatus using the MR head is concerned, when the current density of the sense current is raised, a phenomenon of electromigration resulting from the movement of metal atoms from a cathode to an anode due to collision of conduction electrons takes place. The electromigration may bring about an increase in direct current resistance or a disconnection. A countermeasure against this electromigration is requested.
The service life of an MR head is inversely proportional to the value of a sense current. When a certain sense current is supplied to the MR head, the service life of the MR head is inversely proportional to the time during which the MR head conducts the sense current.
In a magnetic reproduction apparatus using an MR head, therefore, a sense current flowing into the MR head is usually set to the range from 10.sup.6 to 10.sup.7 A/cm.sup.2 in an effort to acquire a large reproduced output from the MR head. As a result, an element in the MR head is heated to a higher temperature than that in an inductive head. Moreover, when the current density of the sense current is high, the electromigration phenomenon takes place. This brings about the fear of deterioration of the magnetic properties of a magnetic film in a head or the fear of a disconnection.
It is effective to suppress electromigration by shortening the time during which a sense current flows unidirectionally in an element in an MR head. A countermeasure against electromigration implemented in a known magnetic disk unit is, for example, as disclosed in Japanese Unexamined Patent Publication No. 5-225531, to reverse the sense current.
However, when the sense current is reversed according to the method disclosed in the Japanese Unexamined Patent Publication No. 5-225531, the direction of the sense current is changed during reproduction of a normal record signal. It is hard to stabilize the reproduction ability of an MR head despite a change in direction of the sense current. This poses a problem that a variation in characteristic of a reproduced signal must be corrected at every reversal of the direction of the sense current.